It is well known to increase the viscosity of water or an aqueous solution by incorporating a polymer as a thickening agent. A number of polymers are known for this purpose including a number of polysaccharides. Viscosity can then be increased further by cross-linking the polymer molecules. This has particular application in connection with the extraction of hydrocarbons such as oil and natural gas from a reservoir which is a subterranean geologic formation by means of a drilled well that penetrates the hydrocarbon-bearing reservoir formation. In this field, one commercially very significant application of thickened fluids is for hydraulic fracturing of the formation. The polymeric thickening agent assists in controlling leak-off of the fluid into the formation, it aids in the transfer of hydraulic fracturing pressure to the rock surfaces and it facilitates the suspension and transfer into the formation of proppant materials that remain in the fracture and thereby hold the fracture open when the hydraulic pressure is released.
Further applications of thickened fluids in connection with hydrocarbon extraction are acidizing, control of fluid loss, diversion, zonal isolation, and the placing of gravel packs. Gravel packing is a process of placing a volume of particulate material, frequently a coarse sand, within the wellbore and possibly extending slightly into the surrounding formation. The particulate material used to form a gravel pack may be transported into place in suspension in a thickened fluid. When it is in place, the gravel pack acts as a filter for fine particles so that they are not entrained in the produced fluid.
Common examples of polymeric thickening agents used in the thickened fluids mentioned above are galactomannan gums, in particular guar and substituted guars such as hydroxypropyl guar. Cellulosic polymers such as hydroxyethyl cellulose may be employed, as well as other carbohydrate based polymers.
Crosslinking of the polymeric materials then serves to increase the viscosity and proppant carrying ability of the fluid, as well as to increase its high temperature stability. Typical crosslinking agents include soluble boron compounds.
The viscosity of these crosslinked gels can be reduced by mechanical shearing (ie they are shear thinning) but gels cross-linked with boron compounds have the advantage that they will reform spontaneously after exposure to high shear. This property of being reversible makes boron-crosslinked gels particularly attractive and they have been widely used.
It is generally desirable to achieve the desired viscosity with a low concentration of thickening materials so as to reduce cost of materials and reduce the amount of material which is delivered below ground and may need to be removed in a subsequent cleanup operation. It is also desirable to minimise the amount of boron which is used.